U.S. Pat. No. 2,865,873, U.S. 3,882,047 and Japanese Pat. No. 47-33082 disclose catalysts that are useful in the oxidation of unsaturated aldehydes to unsaturated acids, wherein ammonia or an ammonium-containing compound is incorporated in the preparation of the catalysts.
U.S. Pat. No. 2,865,873 in Column 13, Examples 101 to 104 discloses a process for the preparation of methacrylic acid using catalysts consisting of molybdenum, phosphorus, titanium and oxygen, wherein ammonium paramolybdate is employed in the preparation of the catalysts. The highest yield of methacrylic acid produced is about 39.56%.
U.S. Pat. No. 3,882,047 discloses the preparation of methacrylic acid using catalysts containing molybdenum, phosphorus, at least one element such as thallium, rubidium, cesium and potassium, and at least one element such as chromium, silicon, aluminum, iron and titanium. This reference teaches the incorporation of ammonia or ammonium-containing compounds in the preparation of all catalysts exemplified in the oxidation of methacrolein or acrolein; phosphomolybdic acid is employed in the preparation of virtually all catalysts exemplified; and in a few examples, ammonia molybdate is employed. This patent discloses in Column 3, lines 30-40 as follows:
"It is preferred that the catalyst be prepared so that the constituent elements will form complex compounds such as heteropolyacids, then acid salts or ammonium salts."
Japanese Pat. No. 47-33082 discloses a process for reclaiming an ammonia-modified phosphorus-molybdenum-X-oxygen catalyst, wherein X is at least one element selected from the group consisting of As, Bi, Si, Cd, W, Tl, Pb, Ge, In, Sn, Sb, and Fe. Preparation of the catalyst involves treating the catalyst with ammonia and water by oxidizing the catalyst in advance or by oxidizing it simultaneously with the treatment of ammonia and water. This patent discloses that the ammonia forms a complex compound with the other elements present.
The present invention is the result of a search for more efficient catalysts for use in the oxidation of acrolein or methacrolein to produce acrylic acid or methacrylic acid, respectively. By the process of the present invention (1) higher yields of methacrylic acid are produced as compared with the art processes; (2) calcination of the catalysts is not required; (3) catalysts are prepared from molybdenum trioxide; and (4) catalysts are prepared in the absence of ammonia or an ammonium-containing compound.